Rhodopsin kinase, an enzyme that catalyzes phosphorylation of rhodopsin with ATP in the light, was identified with a retina-specific soluble antigen (S-antigen) which causes autoimmune uveitis in experimental animals. Rhodopsin kinase, cyclic nucleotide phosphodiesterase and GTP binding protein are localized on the external surface of the disk membrane. These enzymes, together with rhodopsin, form a functional complex. While no GTPase activity is shown by a mixture of purified GTP binding protein, rhodopsin and GTP in the dark, the activity becomes manifested in the light. The N-terminus of rhodopsin was identified as n-acetylmethionine. Evidence is obtained that the chromophore of rhodopsin is protonated by hydronium ion and proton translocation accompanies metarhodopsin I formation.